The MAGT1 transporter is critically involved in the selective regulation of intracellular free Mg2+ levels in mammalian cells. The molecular functions of free Mg2+ in eukaryotic cells have not been established. We found that patients with genetic deficiencies in MAGT1 have high levels of Epstein-Barr virus (EBV) and a predisposition to lymphoma. In studying lymphocytes from these patients, we found that a deficiency of MAGT1 caused decreased basal intracellular free Mg2+ leading to defective expression of the natural killer activating receptor NKG2D in NK and CD8+ T cells. Without NKG2D, cytolytic responses against EBV are diminished, thereby revealing the first specific molecular function of intracellular basal free Mg2+ in eukaryotic cells. Moreover, intracellular free Mg2+, NKG2D expression and function can be rescued in vitro by incubating patient cells and elevated levels of Mg2+. Moreover, NKG2D expression and cytolytic function can be improved and EBV-infected cells reduced in vivo, in MAGT1-deficient patients by magnesium administration. Thus, our data indicate an important molecular function for free basal Mg2+ in immunity and demonstrate a requirement for NKG2D cytolytic function in an essential EBV antiviral response in humans. Recent advances in understanding the role of divalent cations in immune cells have uncovered new signaling functions for Mg2+ and Zn2+ opening new vistas for future investigation. Many questions about Mg2+ signaling are still unanswered. In fact, there are a plethora of apparent Mg2+ transport and channel proteins whose role in human physiology is not yet been determined in the healthy and diseased immune system. Mg2+ mobilization has been observed in variety of cell types, however the molecular mechanisms regulating these cation changes remain elusive. Technical limitations such as probe sensitivity, low electrogenicity, and other considerations prevent the precise study of the regulated mobilization of these cations. Our current collaborations with academic and industrial partners are directed at addressing these issues. Another aspect that require further investigation is the understanding of the various effectors of divalent cations signaling and the network(s) that they generate. Beyond enhancing the basic scientific knowledge, answering these questions have also important medical implications especially, but not only, for immune disorders associated with divalent cations signaling deficiency, such as XMEN disease, SOCE deficiency or even dietary Zn2+ deficiency. Thirty years ago, the discovery of the immunosuppressant abilities of the Ca2+ signaling inhibitors, cyclosporine A (CN inhibitor), revolutionized organ transplantation. Thus, uncovering new Mg2+ and Zn2+ sensitive targets or functions could identify new therapeutic targets for immunomodulation. Moreover, the promising results of Mg2+ and Zn2+ supplementation in various pathological settings should encourage the development of new ion supplementation strategies. Also, there are at least 22 known magnesium transporters or channels of which we have detected at least 16 on T lymphocytes. There is very little known about these transporters or channels so this represents an opportunity to understand new fundamental aspects of ion regulation. Since other ion channels have been good pharmaceutical targets, this may open up new areas for therapeutic development.